This invention relates to an electrophotographic image forming apparatus equipped with a fixing device such as a copying machine, printer, facsimile, and so on. Specifically, this invention relates to an electrophotographic image forming apparatus equipped with a fixing device that fixes by clamping, pressing, and heating a paper sheet by two bodies of rotation.
In general, a conventional fixing device fixes a toner image to a transfer sheet by letting a transfer sheet with a toner image pass through a nip area which is formed by two bodies of revolution (rollers) in pressure contact, heating, and pressing the toner image against the transfer sheet.
When the image forming apparatus is of the color type, the transfer sheet is apt to have more toner and consequently it is apt to twine itself around the body of rotation when a toner image is heated and pressed. To prevent this, the nip area is dented in the side of the unfixed toner image. In this case, the roller which is in contact with the backside of the transfer sheet must be harder than the roller which is contact with the toner image side of the transfer sheet. In this case, the nip area is not wide enough for a poor-fixing transfer sheet such as a cardboard. Therefore, to fix such a poor-fixing transfer sheet, we must increase the fixing temperature or reduce the processing speed. As the result, the warm-up time becomes longer and the print productivity becomes lower. Additionally, after passing through the fixing device, such a hard transfer sheet may be curled to the nip shape. As the cardboard is hard and the nip area need not be convex in the side of the roller facing to the backside of the paper, it is possible to decrease the hardness of the roller in the backside of the transfer sheet and to make the nip area flat. However, in this status, a thin paper sheet may twine itself around the roller in the side of the unfixed toner.
To solve the above problems, there have been disclosed various technologies such as a technology (e.g. Patent Document 1) that uses a plurality of rollers to select optimum conditions such as roller temperatures, diameters, circumferential speeds, and surface hardness according to water content and thickness of the transfer sheets and a technology (e.g. Patent Document 2) that select rollers according to the kinds of transfer sheets to suppress wrinkles of an envelope that holds a toner image and to assure the transparency of a color toner image on an OHT sheet (transparent sheet).
Further, another technology (e.g. Patent Document 3) discloses a method of providing a roller to the unfixed toner image side of a transfer sheet, a belt to the opposite side of the transfer sheet, and a plurality of pressing members that press the belt against the roller, selecting one of the pressing members which have different lengths (widths) perpendicular to the movement of the transfer sheet, and causing the selected pressing member to press the belt against the roller with the pressing force changed.
Patent Document 1: Japanese Non-examined Patent Publication S54-95246
Patent Document 2: Japanese Non-examined Patent Publication H04-166878
Patent Document 3: Japanese Non-examined Patent Publication 2001-5312
However, when some rollers are selected, their temperatures must be controlled simultaneously and the power consumption is required too much in the standby status. If this temperature control is omitted to suppress the standby power consumption, it takes much time before the selected rollers reach the preset control temperatures. In other words, it takes a lot of time for the first printout and the fixing may be insufficient. Further, if controlling is made to reduce the circumferential speeds of rollers, the print productivity becomes lower. Therefore, it is not enough to simply provide rollers that are different in temperature, diameter, circumferential speed, and surface hardness and to select them according to the operating conditions because of the long warm-up time after roller selection and the low print productivity. Furthermore, the technology disclosed by Patent Document 3 cannot assure the fixing and paper passing abilities of various kinds of transfer sheets under a changing print environment singly by changing the length (or width) of the pressing member perpendicular to the movement of the transfer sheet.